In wearable computing, a head mount display (HMD) is known as a display apparatus to be mounted on a human body. When an HMD is mounted on a human's head, the human has his/her hands free. Thus, HMDs are expected as operator assistance display apparatuses.
Main components of an HMD are displays which display a moving image or a still image and eyepieces which expand the range of clear vision of the display. An HMD has one pair of combinations of a display and an eyepiece for each left and right eye, and allows left and right eye to view independent images. The HMD functions as a two-dimensional display when the HMD displays a single same image for both left and right eyes, and it functions as a three-dimensional display which allows depth recognition by binocular parallax when the HMD displays separate images with a parallax therebetween as an image for a left eye and an image for a right eye.
However, regardless of whether an HMD is used, a stereoscopic 3D display utilizing binocular parallax is known to be problematic in that an observer is likely to feel unnaturalness and suffers great eyestrain. As solutions to the problem, a multi-view 3D display system and a super multi-view 3D display system are proposed (see, e.g., Non-patent Document 1).
FIG. 1 shows an example of a multi-view 3D display system. A multi-view 3D display system is a technology to express “motion parallax” that refers to a change in view with a turn of a head by generating a plurality of viewpoint images corresponding to observation positions in a space and displaying 3D images closer to nature. A multi-view 3D display apparatus in FIG. 1 includes a display 101 and a three-dimensional filter 102 and has sets of viewpoint images 1 to 4 placed at intervals corresponding to an inter-ocular distance. With this configuration, a visible stereoscopic image changes according to the observation position, and motion parallax is expressed.
FIGS. 2A and 2B show a difference between a multi-view 3D display system and a super multi-view 3D display system. In the nature world, light rays serving as leads for a human to perceive a three-dimensional space are ones having passed through the pupils of left and right eyes of continuous light rays from an actual object. When such continuous light rays are to be sampled to display a 3D image, for example, a configuration with a lenticular lens 201 provided on a surface of a display is adopted, as in the multi-view 3D display system shown in FIG. 2A. In the lenticular lens 201 in the multi-view 3D display system, however, the sampling interval for viewpoint images to be generated is too wide.
For this reason, it is desirable to make a sampling interval ds for viewpoint images to be generated by a lenticular lens 202 smaller than a pupil diameter dp of a human, as in the super multi-view 3D display system shown in FIG. 2B. Unnaturalness of a stereoscopic image is reduced by, for example, giving a slight parallax between a viewpoint image PR1 and a viewpoint image PR2 corresponding to light rays which are emitted from the lenticular lens 202 and entering an eyeball 203 when an object point P is viewed with a right eye.
FIGS. 3A and 3B show an accommodation of an eyeball which is induced by a super multi-view 3D display system. If the eyeball 203 is focused on the lenticular lens 202, as shown in FIG. 3A, when light rays for a viewpoint image PR1 and a viewpoint image PR2 simultaneously enter the eyeball 203, the two viewpoint images are projected as a double image onto different positions of a retina. In this case, the viewpoint images PR1 and PR2 are recognized as a blur on the retina.
The cerebral function of avoiding blurred vision then induces an accommodative stimulus to the eyeball 203 such that the two viewpoint images are projected onto one point on the retina. If the eyeball 203 is focused on a position in a space which is perceived from a binocular parallax, as shown in FIG. 3B, a natural stereoscopic image is visually recognized.
Similarly, when an observer views an object in the natural world, the accommodation of the eyeball, trying to be focused on a position in a space of an object which is perceived through binocular convergence, works due to the cerebral function of avoiding blurred vision. If eyeballs are focused on a position in a space which is perceived with a binocular parallax between images by the accommodation of the eyeballs, there is no longer any distinction between a state in which an image is viewed and a state in which an object is viewed in the natural world, which allows visual recognition of a natural 3D image.
A configuration of an HMD switchable between 2D display and 3D display is also known in which left and right image light beams are distributed to different directions by providing a lenticular lens between a display and an eyepiece lens (see, e.g., Patent Document 1).
Patent Document 1: Japanese Laid-Open Patent Publication No. 2011-145488
Non-patent Document 1: “Report on the Final Results of the Advanced 3-D Image Telecommunication Project” [online], Telecommunications Advancement Organization of Japan, pp. 144-201, Sep. 9, 1997 [retrieved on Feb. 5, 2013]. Retrieved from the Internet: <URL: http://seika-kokai.nict.go.jp/doc/result/199611005/19961 1005_houkoku-04.pdf>